The present invention relates to an axial fan and, more particularly, to an axial fan having an impeller that can obtain a sufficiently large air flow and air pressure despite its compact, flat shape.
In an axial fan, an impeller is supported to be rotatable about a stator on the main body side of the axial fan. The impeller is formed by equidistantly arranging a plurality of vanes on the outer circumferential surface of a rotor having an annular permanent magnet on its inner circumference. When a rotating magnetic field is generated in the stator, the permanent magnet is attracted thereto so as to rotate the impeller. When the impeller is rotated at a predetermined rotation speed, a desired air flow and air pressure are generated by the vanes. Since the axial fan can be made compact and flat, it is incorporated mainly in more recent electronic equipment, which is also formed compact and flat, so as to prevent an increase in temperature caused by the heat which is generated by internal electronic circuit boards and the like, thereby protecting the electronic elements.
Further downsizing and flattening of the axial fan are demanded in order to cope with recent size reduction of electronic equipment.
The reduction in air flow and air pressure, that naturally arises upon such downsizing and flattening, is not allowable. Therefore, various proposals have been made in order to ensure a large air flow and air pressure.
"A Compact Blower" disclosed in U.S. Pat. No. 5,217,351 is a proposal for performing downsizing and flattening while ensuring an acceptable air flow and air pressure. According to this proposal, extending portions that extend the vanes of the rotor of an axial fan to the central portion of rotation are integrally molded by injection molding using a resin or the like. Due to the function of the extending portions formed in this manner, a decrease in air flow, caused by a decrease in the outer diameter of the vanes of the rotor and in the area of the vanes as the result of downsizing, can be prevented.
According to this proposal, of the plurality of vanes formed on the outer circumferential surface of the impeller of the axial fan, the extending portions formed to extend to the central portion of rotation are continuously molded to form under-molding portions in the mold.
Therefore, the slide piece of the mold that forms the under-molding portions for the vanes is pulled out from the central portion of rotation of the base portion of the impeller in parallel to the radial direction. As a result, the mold becomes rather complicated and accordingly very expensive. A so-called multi-cavity mold for molding a large number of impellers by one injection molding operation has a very complicated structure and is difficult to realize.
When the slide piece that forms the under-molding portion side for the vanes of the impeller is pulled out from the mold in the radial direction, the slide piece cannot be pulled out by twisting. Therefore, it is impossible to form vanes at an angle in accordance with the rotating peripheral velocity of the vanes, and the vanes cannot be formed into an ideal form.
A curved surface is formed on the outer circumferential surface of the base portion of the impeller that forms the under-molding portions for the vanes, so that air supplied by the under-molding portions of the vanes during rotation is supplied to the vanes on the outer circumferential side. The area of the vanes forming the under-molding portions sharply decreases at a portion closer to the central portion of rotation of the base portion. Therefore, the flow of air cannot be reliably captured by the vanes at the under-molding portions formed in this manner.
Since the curved surface is formed on the outer circumferential surface of the base portion of the impeller, the permanent magnet to be incorporated is limited to flat one, and the structure of the rotor is accordingly largely limited.